Toyota Repair: Toyota Highlander Head Gasket, powertrain warranty, coolant level

Question
I have a 2003 4-cylinder Highlander.  The coolant level was low, check engine light was on and the coolant gauge was moving up and down erratically.  I have been told by a mechanic that it is an external head gasket.  The dealer tells me he's never heard of that problem on a Highlander.  What is your experience with this?  What should I expect in repair cost?

Answer
I also agree with the dealer, there have been no headgasket problems on Highlanders at all, even if there was it is still covered under the factory warranty under the powertrain warranty if less than 60,000 miles.
The best advice that I can offer you is to have the toyota dealer diagnose this, there may be a coolant leak or there may be a thermostat problem.
When the check engine light was on did someone retreive the trouble codes and what were they, this is very important, I can be of a lot more help if you could provide that information, without knowing the code why the engine light was on I can't be of much more help.